Proefschrift--Amsterdam--Vrije universiteit, 1995. / Bibliogr. p. 335-352. Index. Résumé en anglais.

Ce travail de thèse traite de la dynamique ultrarapide de spins et de charges dans des oxydes de fer. Dans un premier temps, on montre à l'aide d'un montage pompe-sonde résolu en temps et exploitant l'effet Faraday magnéto-optique, que le temps de désaimantation dans une assemblée de nanoparticules de maghémite est plus rapide que le temps de désaimantation dans une assemblée de nanoparticules de magnétite. Une superposition des temps de thermalisation des électrons et de désaimantation est observée dans la maghémite. Cette accélération du temps de désaimantation est interprétée comme étant la conséquence d'un renforcement des interactions antiferromagnétiques dans la maghémite. La seconde partie prouve qu'il est possible de caractériser la transition de Verwey dans un film de magnétite grâce à des signaux de dynamique de charges et de spins. La dynamique ultrarapide d'aimantation se caractérise par un mouvement de précession dépendant de la température. D'importantes modifications des oscillations sont visibles de part et d'autre de la température de Verwey, reflétant un changement d'anisotropie caractéristique de cette transition. / This work deals with spins and charges ultrafast dynamics in iron oxide. Thanks to a time-resolved magneto-optical Faraday effect measurements, we show the demagnetization time in an assembly of maghemite nanoparticles is faster than the demagnetization time in an assembly of magnetite nanoparticles. A superposition of thermalization times of electron and demagnetization times is observed in maghemite. This acceleration of the demagnetization time is interpreted as the effect of an enhancement of antiferromagnetic interactions in maghemite. The second part demonstrates the possibility to characterize the Verwey transition in a thin film of magnetite thanks to charges and spins dynamics signals. The ultrafast magnetization dynamic shows a temperature-dependent precession motion. Huge modifications of oscillations are visible on both side of Verwey temperature, reflecting an anisotropy change typical of this transition.

This project focused on two aspects of magnetite thin films. The first was to find optimum parameters and conditions for deposition of stoichiometeric Magnetite films using pulsed laser deposition (PLD). The second aspect was the characterization of the magnetic and electrical properties in order to broaden the spectrum of understanding of PLD Magnetite films. These properties were also investigated in terms of the substrates on which the films were deposited. Discussed in this thesis are deposition parameters, structural characteristics, magnetic and electrical characteristics of the films in terms of different substrates and film thicknesses. The discussion consists of structural parameters obtained using X- ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), and electric properties such as resistance as a function of temperature and voltage dependence on the applied current. The magnetic properties measured were the magneto-resistance, M-H hysteresis loop, and magnetization as a function of temperature. The results obtained are then compared with pre-existing literature data. It will be shown that there is an impurity phase that may be seen when magnetite films are deposited on Sillicon dioxide substrates.

Neutron and x-ray diffraction has been used to study charge, orbital and magnetic ordering in some transition metal oxides. The long standing controversy regarding the nature of the ground state (Verwey structure) of the canonical charge ordered material magnetite (Fe3O4) has been resolved by x-ray single crystal diffraction studies on an almost single domain sample at 90 K. The Verwey structure is confirmed to have Cc symmetry with 56 unique sites in the asymmetric unit. Charge ordering is shown to be a useful first approximation to describe the nature of the ground state, and the conjecture that Verwey made in 1939 has finally been confirmed. However, three-site distortions which couple to the orbital ordering of the Fe2+ ordered states (trimerons) are shown to provide a more complete description of the low temperature structure. Trimerons explain the rather continuous distribution of the valence states observed in magnetite below Tv, anomalous shortening of Fe-Fe distances and the off-centre distortions resulting in ferroelectricity. DFT+U electronic structure calculations on the experimental coordinates support the conclusion of this crystallographic study, with the highest electron densities calculated for those Fe-Fe distances predicated to participate in the trimeron bonds. The 6H-perovskites of the type Ba3ARu2O9 have been reinvestigated by high resolution neutron and x-ray power diffraction. The charge ordered state of Ba3NaRu2O9 has been characterised at 110 K (P2/c, a =5.84001(2) Å, b = 10.22197(4) Å, c = 14.48497(6) Å, β = 90.2627(3) °) and shown to consist of a structure with near integer charge ordering of Ru5+ 2O9 / Ru6+ 2O9 dimers. The ground state has been shown to be very sensitive to external perturbations, with a novel melting of charge ordering observed under x-ray irradiation below 40 K (C2/c, a =5.84470(2) Å, b = 10.17706(3) Å, c = 14.45866(5) Å, β = 90.2151(3)-° at 10 K). High pressure studies reveal that the Ru-Ru intra-dimer distance may dictate the response of the system to pressure. Empirical trends in the Ba3ARu2O9 series of compounds have shown that change in ‘chemical pressure’ in these systems may be rationalised in terms of Coulomb’s law. In A = La and Y the magnetic ordering is shown to be FM within the Ru2O9 dimers (1.4(2) μB and 0.5(1) μB, respectively per Ru), representing the first case of intra dimer FM coupling reported in a system containing face-sharing RuO6 octahedra . The overall AFM coupling of the dimers implies an as yet unobserved breaking of the parent symmetry. In A = Nd, a complex competition between the crystal field effect of Nd3+ and the magnetic ordering of the Ru2O9 FM moments has been observed, leading first vi to FM order of Nd at 25 K (1.56(7) μB) followed by ordering of Ru moments (0.5(1) μB) and a spin reorientation transition of Nd moments at 18 K. In A = Ca, the formation of a singlet ground state is observed in Ru2O9 rather than the expected AFM coupling and below 100 K Ba3CaRu2O9 is diamagnetic. All five systems indicate that the Ru2O9 dimer is the physically significant unit in these systems when considering structural trends and the ordering of charge, spin and orbital degrees of freedom.

If a rock is to retain a geologically meaningful magnetic record of its history, it is essential that it contains magnetic minerals which are capable of carrying stable magnetic remanence. Of the natural occurring magnetic minerals, magnetite is the most important because of its abundance and strong magnetic signature. The stability, i.e., the resistance to demagnetisation or reorientation, of magnetic remanence is related to grain size; in smaller grains the magnetic moments align to have single domain (SD) structures, in larger grains complex magnetic patterns are formed (multidomain (MD)). “Classical” domain theory predicts that SD remanence is stable, whilst MD remanence is not. However experimental evidence has shown that both SD and MD grains can have stable remanences. In this thesis the origin of stable MD remanence is examined. There are two opposing theories; one suggests that the stability is due to independent SD-like structures, the other postulates that the stability is due to metastable MD structure. A series of experiments were designed to examine the stability using a selection of characterised synthetic and natural samples. Low-stress hydrothermal recrystallised samples where grown for this study. For the first time, the stability of thermoremanence induced in hydrothermal crystals to cooling was examined. The results agree with previous observations for crushed and natural magnetites, and support kinematic models. The behaviour of SIRM and thermoremanences in MD magnetite to low-temperature cooling to below the crystallographic Verwey transition at 120-124 K (T_v) and the cubic magnetocrystalline anisotropy isotropic point (T_k) at 130 K was investigated. On cooling through T_v, SIRM was observed to decrease and demagnetise, however thermoremanence was found to display a large increase in the magnetisation at T_v, which was partially re- versible on warming. The size of the anomaly is shown to be dependent on the temperature at which the thermoremanence is acquired, internal stress and grain size. The anomaly is attributed to the large increase in the magnetocrystalline anisotropy which occurs on cooling through T_v . It is postulated that low-temperature cycling demagnetisation is due to kinematic processes which occur on cooling between room temperature and T_k. Characterisation of low-temperature treated remanence and partially alternating field demagnetised remanence, suggest that the stable remanence is multidomain. Low-temperature cooling of remanence in single sub-micron crystals was simulated using micromagnetic models. The models predict the observed anomaly for thermoremanence on cooling through T_v, and also the relative behaviour of SIRM and thermoremanence. The single domain threshold was calculated for the low-temperature phase of magnetite, and was found to be 0.14 microns, compared to 0.07 microns at room temperature.

We have performed detailed measurements of magnetic and dielectric properties of high quality magnetite (Fe3O4) single crystals in weak magnetic and electric fields. These measurements can reveal details of phase transitions and other features that are not yet fully understood. We focused not only at the Verwey transition - a well known phase transition taking place at about 120 K in stoichiometric samples - but we also explored and described new relaxation effects in magnetite at low temperatures. The low-temperature properties were also found to be exceptionally sensitive to sample quality, stoichiometry and homogeneity. The results presented in this thesis were acquired on sensitive non-commercial SQUID magnetometer complemented by four-probe dielectric spectroscopy and dc conductivity measurements.

Laboratory magnetic measurements are used to develop a methodology to characterize the Kiruna-type Rare Earth Elements (REE) bearing apatite iron-oxide deposits at Blötberget in central Sweden. This high-grade ore deposit is known to have sharp boundaries between lens shaped main ore bodies of magnetite-rich ore, and a complex hematite-rich ore associated with pegmatites and skarn formation. The thesis covers laboratory magnetic measurements of 37 samples originating from eight drill cores and reference samples from previously mined area. It focuses on on-covering how the samples relate in terms of magnetic susceptibility, further its dependency on temperature, frequency, field and the orientation. The results are correlated with petrographic analysis previously performed on accompanying thin sections. The measurements show that magnetite with strong susceptibility contribution overshadow the hematite contribution in the samples. Transition changes in susceptibility are noticeable when crossing the Verwey temperature; -153°C, Curie temperature; 580°C and Néel temperature; 680°C. The Morin temperature appears at -60°C, or is missing. Linear relationships are identified between the magnitude difference in susceptibilities across transitions at high temperature and wt% magnetite and hematite have been identified. The Blötberget skarn and hematite-rich ore samples have a higher degree of susceptibility anisotropy than the other ore-types. Blötberget samples are dominated by multidomain characteristics in remanence, saturation and coercivity. High temperature measurements have shown that the magnetite is close to pure. The low temperature measurements suggest hematite is impure or bears a petrological footprint. The study also shows that rich iron ore samples sometimes can be at risk of being overlooked with standard methods of measuring susceptibility / Laboratorietekniska metoder kan användas som ett komplement till malmgeologi och geofysisk prospektering. I denna metodstudie karaktäriseras apatitjärnmalm från Blötberget, nära Grängesberg. En fyndighet bestående av linsformade malmkroppar rika på magnetit och ofta avskilda men komplexa hematitrika stråk. Studien är gjord 37 prover från totalt 8 borrkärnor, och lokaler som tillhörde produktion från gruvverksamhet under 1900-talet. Mätmetoderna fokuserar på att kartlägga malmens magnetiska egenskaper, och hur temperatur, frekvens, fältstyrka samt riktning påverkar dessa. Resultaten jämfördes med tidigare petrografisk studie av tillhörande tunnslip Resultaten visar att magnetit står för merparten av susceptibiliteten i proverna, men att även hematit kan urskiljas och kvantifieras. Temperaturberoende har påvisats vid övergångar för Verwey-temperatur; -153°C, Curie-temperatur; 580 °C, och Néeltemperatur; 680 °C. Den förväntade Morin-temperaturen vid -14°C, påträffades vid -60 °C eller saknas helt för flera av de hematitrika proverna. Magnetiskt anisotropa prover återfinns bland prover som identifierats som skarn eller hematitrika. Magnetisk granulometri visar karaktär av multidomäntyp med låg magnetisk coercivitet och hög satureringsförmåga. Högtemperaturmätningar av susceptibilitet visar på ren magnetit för prover från Blötberget. Samtidigt visar lågtemperaturemätningar att hematit sannolikt har inblandning av titanium eller bär på ett mer komplext förflutet. Studien visar också att det finns en risk i att enbart förlita sig på bulksusceptibilitet för prover rika på malm.

La magnétite Fe3O4 est un matériau prometteur pour l'électronique de spin, puisque des calculs de structure de bande la prédisent demi-métallique, ce qui laisse supposer des effets de magnétorésistance importants pour des jonctions tunnel magnétiques utilisant Fe3O4 comme électrode. Nous avons fait croître des films minces de Fe3O4 de 3 à 50 nm d'épaisseur sur α-Al2O3 par épitaxie par jets moléculaires. Les films sont monocristallins mais contiennent des parois d'antiphase. Celles-ci sont responsables des anomalies magnétiques observées sur les films, que nous avons pu reproduire à l'aide d'un modèle unidimensionnel, les résultats ayant étés confrontés avec la taille caractéristique des parois mesurée par une analyse<br />fractale. <br /><br />Nous avons de plus mis au point la croissance de couches minces de γ-Al2O3 épitaxiée sur les films de Fe3O4, en contrôlant la stoechiométrie à l'interface entre oxydes. Les films de γ-Al2O3 d'épaisseur supérieure à 2 nm sont continus, et assurent le découplage magnétique entre les électrodes. Les mesures directes par photoémission résolue en spin conduisent à une polarisation en spin de -40 % pour l'interface Fe3O4/ γ-Al2O3 tandis que l'on mesure<br />une magnétorésistance tunnel, réduite du fait du désordre magnétique induit par les parois d'antiphase, de +3 % à température ambiante pour les jonctions Fe3O4/ γ-Al2O3/Co, ce qui implique une polarisation effective positive pour l'interface Fe3O4/ γ-Al2O3 dans les mesures de transport tunnel.